Sinusoidal currents and voltages of a multiphase ac electrical system are commonly represented as a set of rotating vectors. These vectors which may be unsymmetrical or unbalanced can be resolved into three symmetrical systems of three equal vectors each which are known as positive, negative and zero sequence components. The detection of these components is useful in determining the degree of unbalance in the phases of the electrical system and can be used to indicate the occurrence of a fault in the system. With a balanced system, normally only positive sequence components are present. Negative sequence components occur if there is a phase unbalance while zero sequence components only flow if there is a four wire connection such as a grounded wye. In general, the positive sequence components are used in many control and metering applications because these components represent the real portion of the total applied voltage, current, or power. For example, in a motor, forward torque is a function of the positive sequence voltage and the positive sequence current. In power regulators, the positive sequence components are measured to determine the real power applied. One means currently in use for measuring and detecting positive sequence components is the use of a positive sequence bandpass filter. Such a filter is described in U.S. Pat. No. 4,479,160 issued Oct. 23, 1984 and entitled "Bandpass Sequence Filters for Symmetrical Components of Multiphase AC Power Systems" and assigned to the assignee of the present invention. One problem in using the output of such a filter for measuring and metering applications is that the amplitude and phase of the outputs of the filter are sensitive to unbalance in the inputs. This means that any phase or amplitude unbalance in the input is partially carried through to the filter.
A stable timing waveform which would remain precisely phase locked to the symmetrical positive phase sequence components of a source signal, even if the presence of severe distortion and disturbances, would be useful. Further, a circuit in which the positive sequence component output will remain phase locked even though all but one of the phases of the source signal are lost would also be useful. It is the object of the invention to produce a stable phase locked set of symmetrical positive or negative sequence outputs even during the presence of severe unbalance or loss of all the source phases but one. It is a further object of the invention to provide a circuit in which the symmetrical component output may be phase locked to any set of the symmetrical components occurring in the source signal.